Vertical Semiconductor Device with Thinned Substrate

ABSTRACT

A vertical semiconductor device is formed in a semiconductor layer having a first surface, a second surface and background doping. A first doped region, doped to a conductivity type opposite that of the background, is formed at the second surface of the semiconductor layer. A second doped region of the same conductivity type as the background is formed at the second surface of the semiconductor layer, inside the first doped region. A portion of the semiconductor layer is removed at the first surface, exposing a new third surface. A third doped region is formed inside the semiconductor layer at the third surface. Electrical contact is made at least to the second doped region (via the second surface) and the third doped region (via the new third surface). In this way, vertical DMOS, IGBT, bipolar transistors, thyristors, and other types of devices can be fabricated in thinned semiconductor, or SOI layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/270,339, filed Oct. 11, 2011, published as U.S. PatentPublication 2012/0088339, which claims priority to U.S. ProvisionalPatent Application No. 61/392,419 filed Oct. 12, 2010, both of which arehereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

Semiconductor power devices have been in use since the early 1950s. Theyare specialized devices used as switches or rectifiers in powerelectronics circuits. Semiconductor power devices are characterized bytheir ability to withstand high voltages and large currents as well asthe high temperatures associated with high power operation. For example,a switching voltage regulator will comprise two power devices thatconstantly switch on and off in a synchronized manner to regulate avoltage. The power devices in this situation need to sink system-levelcurrent in the on state, withstand the full potential of the powersupply in the off state, and dissipate a large amount of heat. The idealpower device is able to operate in high power conditions, can rapidlyswitch between on and off states, and exhibits low thermal resistance.

A standard power device structure implemented using Metal-OxideSemiconductor Field Effect Transistor (MOSFET) technology is theVertical Diffused Metal-Oxide Semiconductor (VDMOS) structure. The VDMOSstructure is also known as Double-diffused MOS (DMOS). The “vertical”term is used because current flows vertically through the device, andthe “diffused” term is used because the channel and source regions areproduced through a diffusion processing step. The structure can bedescribed with reference to FIG. 1.

FIG. 1 displays a cross-section of a VDMOS power device 100. The powerdevice 100 includes one or more source electrodes 101, a drain electrode102, and a gate electrode 103. Source regions 104 are N+ doped in ann-type VDMOS device. In contrast to a standard MOSFET configuration, thesource regions 104 are located on either side of a gate 105 below a gateinsulator 106. Channel regions 107 are P+ doped in an n-type VDMOSdevice, and they are disposed between a drain region 108 and the sourceregions 104. In an n-type VDMOS device a high voltage applied to thegate electrode 103 will invert the channel regions 107 between thesource regions 104 and the drain region 108. This configuration allowsthe power device 100 to withstand both a high voltage in the off stateand a high current in the on state as compared to a standard MOSFETimplemented using the same amount of die area. The channel width of thepower device 100 is double that of a traditional MOSFET with the samedie area thereby allowing the power device 100 to withstand largecurrents. In addition, the dimension that would usually be the channellength in a traditional MOSFET does not affect the breakdown voltage.Instead, the thickness and doping of the drain region 108 determines thebreakdown voltage of the power device 100. The drain region 108 isusually the device substrate when a VDMOS device is implemented in aregular bulk semiconductor process.

The VDMOS power device 100 has certain disadvantageous aspects thatlimit it from performing as an ideal power device. For instance, thereis a large junction capacitance formed by the boundary between the drainregion 108 and the channel region 107. This capacitance is generally dueto an area component set by a dimension 111 and a depth component set bya dimension 110. Since the junction formed by the drain region 108 andthe channel region 107 must be charged or discharged when the powerdevice 100 switches state, the capacitance of this junction degrades theperformance of the power device 100. In addition, since the areacomponent is limited, it is not possible to contact the source regions104 and the channel regions 107 separately, since electrodes such assource electrode 101 can often consume a large amount of area.Furthermore, the power device 100 suffers from very poor thermalperformance, since it is implemented on bulk semiconductor. Powerdevices implemented in bulk semiconductor typically have a minimum waferthickness of approximately 200 μm due to the high incidence of waferbreakage when handling large-diameter wafers thinner than that. Sincethe thermal resistance of a silicon substrate is proportional to thethickness of the silicon substrate, the implementation of power deviceson bulk semiconductor is problematic in terms of thermal performance. Ahigh level of heat in an integrated circuit can shift the electricalcharacteristics of its devices outside an expected range causingcritical design failures. Left unchecked, excess heat in a device canlead to permanent and critical failures in the form of warping ormelting materials in the device's circuitry.

Additionally, layer transfer technology typically involves a pair ofsemiconductor wafers at various stages of processing that are bondedtogether using direct, molecular, or adhesive bonding. If one of thewafers is a semiconductor-on-insulator (SOI) or silicon-on-insulatorwafer with the substrate removed to expose the buried oxide, theresulting structure comprises a device layer that is upside-down withrespect to its original orientation and that has been transferred froman SOI wafer to a new handle wafer.

A layer transfer structure 200 is shown in FIG. 2. The layer transferstructure 200 includes a handle wafer 201 and an SOI wafer 202. Thehandle wafer 201 comprises a handle wafer substrate 203 and a handlebond layer 204. The SOI wafer 202 comprises an insulator layer 205 and acircuitry layer 206. The layer transfer structure 200 illustrates thefinished product of a layer transfer process. However, before layertransfer begins, the SOI wafer 202 additionally comprises another layerof substrate material below the insulator layer 205. The substrate layeris typically a semiconductor material such as silicon. The insulatorlayer 205 is a dielectric which is often silicon-dioxide formed throughthe oxidation of the substrate silicon. The circuitry layer 206 includesa combination of dopants, dielectrics, polysilicon, metal layers,passivation, and other layers that are present after structures 207 havebeen formed therein. The structures 207 may include metal wiring;passive devices such as resistors, capacitors, and inductors; and activedevices such as transistors. Layer transfer begins when the handle bondlayer 204 is bonded to the top of the SOI wafer 202. At this point, thehandle wafer 201 provides sufficient stability to the SOI wafer 202 suchthat the aforementioned layer of substrate material below the insulatorlayer 205 can be removed. As a result of this process, the layertransfer structure 200 provides a device that can be contacted through abottom surface 208. This means that external contacts to the structures207 in the circuitry layer 206 are extremely close to the structures 207themselves. In some situations this distance is on the order of 1micro-meter (μm).

As used herein and in the appended claims, the “top” of the layertransfer structure 200 references a top surface 209 while the “bottom”of the layer transfer structure 200 references the bottom surface 208.This orientation scheme persists regardless of the relative orientationof the circuitry layer 206 to other frames of reference, and the removalof layers from, or the addition of layers to the SOI wafer 202.Therefore, the circuitry layer 206 is always “above” the insulator layer205. In addition, a vector originating in the center of the circuitrylayer 206 and extending towards the bottom surface 208 will always pointin the direction of the “back side” of the layer transfer structureregardless of the relative orientation of the SOI wafer 202 to otherframes of references, and the removal of layers from, or the addition oflayers to the SOI wafer 202.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a cross-section of a prior art VDMOSpower device.

FIG. 2 is a simplified diagram of a cross-section of a prior art layertransfer structure.

FIG. 3 is a simplified diagram of a cross-section of a vertical powerdevice incorporating an embodiment of the present invention.

FIG. 4 is a simplified diagram of a cross-section of a vertical powerdevice incorporating an alternative embodiment of the present invention.

FIG. 5 is a simplified diagram of an example plan layout pattern for avertical power device incorporating an embodiment of the presentinvention.

FIG. 6 is a simplified diagram of another example plan layout patternfor a vertical power device incorporating an embodiment of the presentinvention.

FIG. 7 is a simplified diagram of a cross-section of a vertical powerdevice incorporating another alternative embodiment of the presentinvention.

FIG. 8 is a simplified diagram of another example plan layout patternfor a vertical power device incorporating an embodiment of the presentinvention.

FIG. 9 is a simplified diagram of another example plan layout patternfor a vertical power device incorporating an embodiment of the presentinvention.

FIG. 10 is a simplified diagram of a cross-section of a vertical powerdevice incorporating another alternative embodiment of the presentinvention.

FIG. 11 is a simplified diagram of a cross-section of an Insulated GateBipolar Transistor (IGBT) device incorporating another alternativeembodiment of the present invention.

FIG. 12 is a simplified diagram of a cross-section of a vertical bipolartransistor device incorporating another alternative embodiment of thepresent invention.

FIG. 13 is a simplified diagram of a cross-section of a UMOS deviceincorporating another alternative embodiment of the present invention.

FIG. 14 is a simplified diagram of a cross-section of another UMOSdevice incorporating another alternative embodiment of the presentinvention.

FIG. 15 is a simplified diagram of a cross-section of a Gate Turn Off(GTO) Thyristor device incorporating another alternative embodiment ofthe present invention.

FIGS. 16 a-b are simplified diagrams of cross-sections of a layertransfer device having a vertical power device incorporating anotheralternative embodiment of the present invention.

FIG. 17 is a simplified diagram of a cross-section of a semiconductordie having multiple devices and incorporating an embodiment of thepresent invention.

FIG. 18 is a simplified flow chart for a process for fabricating one ormore of the devices shown in FIGS. 3-10, 13, 14, 16 and/or 17, accordingto an embodiment of the present invention.

FIG. 19 is a simplified flow chart for a process for fabricating one ormore of the devices shown in FIGS. 11 and/or 12, according to anembodiment of the present invention.

FIG. 20 is a simplified flow chart for a process for fabricating one ormore of the devices shown in FIG. 15, according to an embodiment of thepresent invention.

FIG. 21 is a simplified flow chart for a process for fabricating devicesshown in FIGS. 22 through 26, according to an embodiment of the presentinvention.

FIGS. 22A-H show cross-sectional drawings at several stages ofprocessing of a DMOS device according to an embodiment of the presentinvention.

FIG. 23 shows a cross-sectional drawing of an IGBT according to anembodiment of the present invention.

FIG. 24 shows a cross-sectional drawing of a device with a P+ connectionon its rear side, according to an embodiment of the present invention.

FIGS. 25A-E show cross-sectional drawings at several stages ofprocessing of a DMOS device, using an SOI structure, according to anembodiment of the present invention.

FIG. 26 shows a cross-sectional drawing of an SOI device with the entiresubstrate removed, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in several related exampleembodiments described below. Each embodiment generally exhibitsimprovements in the performance metrics described in the background,e.g. electrical performance improvements in the ability to transitionrapidly from an off state to an on state and thermal performanceimprovements in the ability to dissipate large amounts of heat. Inaddition, some of the embodiments enable additional benefits from theability to independently bias the source and body of a power transistor.Additionally, some of the embodiments achieve some of the improvementsor benefits by including layer transfer structures and techniques.Furthermore, some of the improvements or benefits are enabled bythinning the semiconductor substrate, whether using an SOI(semiconductor on insulator) or bulk semiconductor wafer, and with orwithout layer transfer structures and techniques. Also, some embodimentsachieve some improvements by including an isolating trench around theactive regions, which also benefits from the thinning of thesemiconductor substrate to more thoroughly isolate the active regions.Additionally, some embodiments achieve some improvements by enabling theability to integrate any desired number and combination of independentvertical semiconductor devices described herein (including multiplevertical power devices, among others) on one integrated circuit (IC)chip or die along with (or without) other additional analog or digitalcircuitry, including embodiments that do not have to form common drainsfor all of the devices via a common substrate. Furthermore, although thesemiconductor material in many embodiments may be described herein assilicon, it is understood that the present invention is not necessarilyso limited, but that other semiconductor materials (e.g. GaAs, SiC, GaN,InGaAs, InP, etc.) are generally within the scope of the presentinvention.

Reference now will be made in detail to some embodiments of thedisclosed invention, one or more examples of which are illustrated inthe accompanying drawings. Each example is provided by way ofexplanation of the present technology, not as a limitation of thepresent technology. In fact, it will be apparent to those skilled in theart that modifications and variations can be made in the presenttechnology without departing from the spirit and scope thereof. Forinstance, features illustrated or described as part of one embodimentmay be used with another embodiment to yield a still further embodiment.Thus, it is intended that the present subject matter covers all suchmodifications and variations within the scope of the appended claims andtheir equivalents.

Some embodiments of the present invention generally provide for verticalpower devices having low parasitic capacitance, low thermal resistance,and high isolation. Some embodiments of the present invention achievethese beneficial results by eliminating portions, or minimizing avertical and/or horizontal dimension, of the drain region 108 (FIG. 1)such that the capacitance between the drain and channel regions in thenew devices is significantly reduced. In some embodiments of the presentinvention, the portion of the drain region 108 that is removed (e.g. dueto thinning of the semiconductor substrate) is the portion of the drainregion 108 below the channel region 107 such that the channel area inthe new devices is generally decoupled from the internal capacitance ofthe new device. Some embodiments of the present invention utilize theresulting decoupling of the channel area and device performance torealize additional benefits such as providing a VDMOS for which the bodyand source can be connected independently without die area penalty.Additionally, some embodiments of the present invention provide for lowthermal isolation by reducing the distance from the active,heat-generating region of the device to the back side of the device toapproximately 1 um (i.e. thinning the semiconductor substrate) so thatthe most rapid thermal path is provided for the active regions of thedevice. In addition, some embodiments of the present inventionimplementing an NMOS and/or PMOS power device achieve the aforementionedbeneficial results by redefining the roles of the drain and sourceregions such that the top electrode is connected to the drain region anda single back side contact connects to both the source and channelregions.

Some embodiments of the present invention can be described withreference to FIG. 3. FIG. 3 illustrates a vertical power device 300 thatmay form part of an overall IC chip and that is in accordance with thepresent invention. In various embodiments of the present invention, thevertical power device 300 is preferably either an NMOS or a PMOSvertical transistor. The vertical power device 300 generally comprisesfirst, second and third semiconductor regions 301, 302 and 303 within anactive surface layer, or active semiconductor region. (As used hereinand in the appended claims, the “active layer” or “active semiconductorregion” refers to the part of a semiconductor substrate in whichsemiconductor structures have been implanted, doped or deposited.) Thevertical power device 300 also has a gate region 304 over the activelayer. The first semiconductor region 301 is generally below the gateregion 304, which is surrounded by an oxide/insulator 305. The secondsemiconductor region 302 is preferably of the same or similar electricaltype as the first semiconductor region 301. (For example, if thevertical power device 300 is an n-type device then first and secondsemiconductor regions 301 and 302 are n-type regions.) The thirdsemiconductor region 303 generally isolates the first semiconductorregion 301 from the second semiconductor region 302. The thirdsemiconductor region 303 has a bottom boundary 306 and a side boundary307 that extends downward from the gate region 304 to the bottomboundary 306. The first semiconductor region 301 contacts the thirdsemiconductor region 303 along the side boundary 307 and does notcontact the third semiconductor region 303 along the bottom boundary306. In other words, compared to the prior art drain region 108 of FIG.1, vertical and horizontal dimensions of the first semiconductor region301 have been minimized (e.g. to minimize parasitic capacitance, thermalresistance and electrical resistance). Additionally, the thirdsemiconductor region 303 is preferably electrically complementary to thefirst semiconductor region 301 and the second semiconductor region 302.(For example, if the vertical power device 300 is an n-type device thenthe third semiconductor region 303 is p-type.)

In some embodiments of the present invention, the bottom boundary 306 isdisposed on a buried insulator layer (not shown) of an SOI (or bulksemiconductor) substrate and is substantially normal to a line drawndirectly from top electrode 308 to the back side of the wafer containingthe vertical power device 300. In some embodiments of the presentinvention, a buried oxide layer is disposed on the back side of thefirst semiconductor region 301 and may also be disposed on the back sideof the third semiconductor region 303. In addition, the buried oxidelayer may be absent in certain locations to provide a back side contact(e.g. bottom side drain electrode 309) to either of these semiconductorregions 301 and/or 303.

In some embodiments of the present invention, the vertical power device300 will comprise the single-gate structure shown and will be isolatedby a trench oxide or shallow trench isolation (STI) region 310. However,a single power transistor is often comprised of many such single-gatestructures. Each of these single gate structures is called a finger.Multiple fingers or multiple power devices may thus share (i.e. besurrounded by) the same trench oxide 310. Alternatively, an array ofsuch power transistors may be created, each separated by the trenchoxide 310.

The trench region 310 preferably extends along an entire vertical side311 of the third semiconductor region 303. Thus, the trench region 310generally penetrates through the entire active layer of the verticalpower device 300. Additionally, the trench region 310 generallyhorizontally surrounds an entire active area of the vertical powerdevice 300 (or the multiple fingers or the multiple power devices ofwhich the vertical power device 300 is a part). The active area thussurrounded is generally electrically isolated from other active areas ofother power devices or transistors on the same die. (The trench region310, thus, generally eliminates the need to form devices with commondrains in a common substrate, since the substrate is generally removedor thinned to the point that the trench region 310 completely (or almostcompletely) electrically isolates each device on the overall IC chip.)The manufacturing or fabrication process (including thinning of thesemiconductor substrate) generally enables this feature for this andother embodiments of the present invention, as described below.

In some embodiments of the present invention, the first semiconductorregion 301 serves as the drain of the vertical power device 300, thesecond semiconductor region 302 serves as the source of the verticalpower device 300, and the third semiconductor region 303 serves as thebody or channel region of the vertical power device 300. In someembodiments having this configuration of source and body, a singleelectrode such as top electrode 308 can be connected to both the thirdsemiconductor (body/channel) region 303 and the second semiconductor(source) region 302 as certain benefits accrue from connecting the bodyand source in a power transistor device.

Several benefits accrue to embodiments of the present invention that arein accordance with the principles taught by FIG. 3. For instance, thejunction between the third semiconductor region 303 and the firstsemiconductor region 301 forms one of the largest capacitances that mustbe charged and discharged when the vertical power device 300 switchesbetween an on and off state. As such, the fact that no portion of thefirst semiconductor region 301 is below the bottom boundary 306 of thethird semiconductor region 303 significantly reduces the capacitance ofthis junction and therefore increases the speed of the vertical powerdevice 300. If the first semiconductor region 301 is used as the drainof the vertical power device 300, these embodiments effectivelyeliminate or minimize most or all of the area component of thebody-to-drain capacitance and leave only the sidewall component, therebyresulting in lower parasitic capacitance and therefore higherperformance. An additional benefit that accrues from the decoupling ofthe size of the horizontal area of the third semiconductor region 303and the performance of the vertical power device 300 is that the thirdsemiconductor region 303 can have a larger horizontal area and thereforelower resistance from the top electrode 308. Since it is advantageous tocontrol the voltage of the third semiconductor region 303, a lowerresistance is beneficial because the voltage will stay consistentthroughout the extent of the third semiconductor region 303 and can bemore accurately controlled. In some embodiments of the presentinvention, this advantageous aspect can also improve the breakdownvoltage of the vertical power device 300 and leakage from the firstsemiconductor region 301 to the second semiconductor region 302.

Some embodiments of the present invention can be described withreference to FIG. 4. FIG. 4 displays a vertical power device 400 thatmay form part of an overall integrated circuit (IC) chip and that is inaccordance with the present invention. The vertical power device 400generally comprises first, second and third semiconductor regions 401,402 and 403 within an active surface layer. The vertical power device400 also generally comprises a gate region 404 (surrounded by adielectric 405). The third semiconductor region 403 isolates the firstand second regions 401 and 402. Similar to embodiments in accordancewith FIG. 3, no portion of the first semiconductor region 401 is belowthe third semiconductor region 403, i.e. vertical and horizontaldimensions of the first semiconductor region 401 have been minimized(e.g. to minimize parasitic capacitance, thermal resistance andelectrical resistance).

A dimension 406 (length of the third semiconductor region 403) is muchlarger in FIG. 4 as compared to a corresponding dimension 312 in FIG. 3.However, this difference does not limit the performance of the verticalpower device 400 because the area of the third semiconductor region 403has been decoupled from the internal capacitance of the vertical powerdevice 400. Therefore, the second semiconductor region 402 can beconnected to a top electrode 407 and the third semiconductor (channel)region 403 can be connected separately to a back (or bottom) sidechannel electrode 408 without increasing the size of the overall ICchip.

Also, due to this configuration, the area of the third semiconductorregion 403 that is available for contacting the back side channelelectrode 408 may be greater than is conventional without significantlyincreasing the size of the overall IC chip. A larger contact size hasthe benefit of decreasing the resistance between the third semiconductorregion 403 and the back side channel electrode 408.

Additionally, due to this configuration, the back side channel electrode408 can be placed on the third semiconductor region 403 as close aspossible to the portion of the third semiconductor region 403 that isclosest to a gate region 404 and directly between the first and secondsemiconductor regions 401 and 402. In this manner, resistance is furtherreduced.

Several benefits accrue to embodiments of the present invention that arein accordance with the principles taught by FIG. 4. In embodimentswherein the third semiconductor region 403 is the body/channel region ofthe vertical power device 400, this body region can be more directlycontrolled because the voltage biasing the second semiconductor region402 is now independent of the body's bias voltage. In addition, inembodiments where the second semiconductor region 402 is the source ofthe vertical power device 400, the fact that the channel and source canbe biased independently allows for the formation of a dynamic thresholdMOS (DTMOS) transistor. The threshold voltage of a DTMOS transistor ismodified using the body effect to bring about beneficial electricalperformance. When a DTMOS transistor is off, the threshold voltage ofthe transistor can be set high through control of the body voltageresulting in very low leakage currents and a high breakdown voltageduring the off state. When the transistor is in the on state anincreased body voltage reduces the threshold voltage, thereby increasingthe current flowing through the transistor in all regions of operation.This improved current flow results in improved power transistorefficiency.

Another benefit of these embodiments is that the separate contacts tothe first and third semiconductor regions 401 and 403 offer a lowthermal resistance path for heat that is built up in the active regionof the vertical power device 400. The back side channel electrode 408and a bottom side electrode 409 (connected to the first semiconductorregion 401) are routed using metal with much lower thermal resistance ascompared to bulk semiconductor or any buried oxide that may be disposedon the backside of the vertical power device 400. Also, since theconnection to the third semiconductor region 403 is not routed upthrough the vertical power device 400 before providing a path out of theoverall IC chip, the path for heat dissipation is much shorter and istherefore more efficient. Typical substrate thicknesses for bulkvertical power devices are about 200 μm. However, the semiconductorthickness for the vertical power device 400 shown in FIG. 4 is about 1μm (e.g. due to thinning of the semiconductor substrate). The resultingdistance from the heat-generating active region to the metal contact onthe back of the overall IC chip is thus reduced by approximately 99.5%from the previous value, with thermal resistance of the semiconductorlayer similarly reduced by 99.5%.

In some embodiments of the present invention, all of the electrodes inFIG. 4, including the top electrode 407 and an electrode for the gateregion 404, in addition to the back side channel electrode 408 and thebottom side electrode 409, can be contacted on the back side of thewafer containing the vertical power device 400. To create thisconfiguration using only one layer of metal routing above the activeregion, the electrodes connected to the top side of the vertical powerdevice 400 can be routed through a plane extending out of the page. Insome embodiments of the present invention, any combination of back andfront side contacts can be used to provide optimal thermal resistancefor heat performance and optimal series resistance for accurate biasconditions. In addition, back and front side contacts can be mixed asshown in FIG. 4 to save space as contacts to the third semiconductorregion 403 and the second semiconductor region 402 can be located in thesame vertical slice of the wafer containing the vertical power device400.

In some embodiments of the present invention, the body of the DTMOSformed by vertical power device 400 can be routed out and connected toanother circuit element that will bias the body when the transistor isturned on and off. For example, the body bias could be 0 or −2V when a2.5 V power supply transistor is off, and 0.6 V when the gate is turnedon at 2.5 V. This increases the body voltage when the gate voltage isincreased, but not enough to forward bias the body with respect to thesource and drain. This is beneficial given that the gate voltage shouldbe as high as possible for low R_(on) and high drive strength. Thisallows enhanced performance without forward bias problems.

Also shown in FIG. 4 is a trench region 410 (e.g. similar to trenchregion 310 of FIG. 3). The trench region 410 preferably extends along anentire vertical side 411 of the third semiconductor region 403. Thus,the trench region 410 generally penetrates through the entire activelayer of the vertical power device 400. Additionally, the trench region410 generally horizontally surrounds an entire active area of thevertical power device 400 (or the multiple fingers or the multiple powerdevices of which the vertical power device 400 is a part). The activearea thus surrounded is generally electrically isolated from otheractive areas of other power devices or transistors on the same die. Themanufacturing or fabrication process (including thinning of thesemiconductor substrate) generally enables this feature for this andother embodiments of the present invention, as described below.

Some embodiments of the present invention can be described withreference to FIG. 5. FIG. 5 displays a top view of a potential layoutpattern for a vertical power device 500, having two fingers 501 and 502,that may form part of an overall integrated circuit (IC) chip and thatis in accordance with the present invention. FIG. 5 will be describedwith reference to an n-type vertical power device with a drain regionthat is back side contacted. However, a similar layout pattern will workfor a p-type vertical power transistor and for a vertical powertransistor having a drain region on the top side. The two fingers 501and 502 generally comprise gate electrodes 503 that are coupled topoly-silicon running along gate regions 504. The gate regions 504 covera strip of n-type material that forms the first semiconductor region 301and 401 in FIGS. 3 and 4 which in this case is the drain of the verticalpower device 500. The gate regions 504 may also cover a portion of thethird semiconductor region 303 and 403, which in this case is thechannel region and is preferably p-type material. Source regions 505generally comprise the second semiconductor regions 302 and 402 fromFIGS. 3 and 4. These regions 505 are also preferably n-type in thiscase. The source regions 505 cover strips of p-type material whichcomprise the channel region of the vertical power device 500. Exposedchannel regions 506 are p-type material which also comprises a portionof the channel region of the vertical power device 500. However, theexposed channel regions 506 are left uncovered so that they may becontacted from the top. Regions 505 and 506 are each made large enoughor wide enough to provide an area for an electrical contact withoutdirectly diminishing the performance of the vertical power device 500because, as described above, dimension 406 (FIG. 4) can be expandedwithout increasing any critical internal capacitance of the verticalpower device 500. Additionally, the entire structure of the verticalpower device 500 is preferably horizontally surrounded by the trenchregion 310 or 410 (FIGS. 3 and 4) to electrically isolate the verticalpower device 500 from other active areas of other power devices ortransistors on the same die.

Some embodiments of the present invention can be described withreference to FIG. 6. FIG. 6 displays a top view of a potential layoutpattern for a vertical power device 600 that may form part of an overallintegrated circuit (IC) chip and that is in accordance with the presentinvention. FIG. 6 will be described with reference to an n-type verticalpower device with a drain region connected on the back side. However, asimilar layout pattern will work for a p-type vertical power transistor.The vertical power device 600 generally comprises a gate electrode 601which is coupled to poly-silicon running along a gate region 602. Thegate region 602 covers a strip of material that forms the firstsemiconductor region 301 and 401 in FIGS. 3 and 4, which in this case isthe n-type drain of the vertical power device 600. Source regions 603cover strips of material that comprise the channel region of thevertical power device 600, which in this case is p-type. Exposed channelregions 604 are p-type material, which also comprises a portion of thechannel region of the vertical power device 600. The exposed channelregions 604 have been left uncovered by the n-type material in thesource regions 603. These regions 604 can be made large enough to allowfor electrical contact to the channel region in these locations. Thislayout is generally more space-efficient than the layout shown in FIG.5, but the series resistance of the body voltage will be slightlyhigher, since there are regions of the channel (e.g. along the length ofthe source regions 603) that will be relatively far from the channelcontacts. Additionally, the entire structure of the vertical powerdevice 600 is preferably horizontally surrounded by the trench region310 or 410 (FIGS. 3 and 4) to electrically isolate the vertical powerdevice 600 from other active areas of other power devices or transistorson the same die.

Some embodiments of the present invention can be described withreference to FIG. 7. FIG. 7 displays a vertical power device 700 thatmay form part of an overall integrated circuit (IC) chip and that is inaccordance with the present invention. The vertical power device 700generally comprises first, second and third semiconductor regions 701,702 and 703 within an active surface layer. The vertical power device700 also has a gate region 704 (surrounded by an oxide/insulator 705)over the active layer. The third semiconductor region 703 isolates thefirst semiconductor region 701 from the second semiconductor region 702.The first semiconductor region 701 and the second semiconductor region702 are generally the same type of semiconductor material, and the thirdsemiconductor region 703 is a complementary semiconductor material. Adistinguishing feature of embodiments illustrated by FIG. 7 is thatthere is a portion of the third semiconductor region 703 (extending outof the plane of the page) that is laterally and vertically coextensivewith the second semiconductor 702 and mutually exclusive with the firstand second semiconductor regions 701 and 702. In other words, in someembodiments of the present invention generally involving thinning of thesemiconductor substrate, not only is the material that comprises thefirst semiconductor region 701 completely absent from below both thethird and second semiconductor regions 703 and 702, but the materialthat comprises the third semiconductor region 703 is also completelyabsent from below the second semiconductor region 702. In other words,vertical and/or horizontal dimensions of the first and thirdsemiconductor regions 701 and 703 have been minimized (e.g. to minimizeparasitic capacitance, thermal resistance and electrical resistance).

Variations discussed above with reference to FIGS. 3 and 4 can beapplied to embodiments taught by FIG. 7. For example, the vertical powerdevice 700 can implement an n-type power device if the secondsemiconductor region 702 and the first semiconductor region 701 aren-type and the third semiconductor region 703 is p-type. However, thevertical power device 700 can also implement a p-type power device.

Several benefits generally accrue to embodiments of the presentinvention that are in accordance with the principles taught by FIG. 7.As seen in FIG. 7, generally due to thinning of the semiconductorsubstrate and further backside processing, the second semiconductorregions 702 can be contacted on either the front or the back sidethrough top electrodes 706 or bottom electrodes 707, respectively.Similarly, the third semiconductor regions 703 can be contacted oneither the front or back side through top or bottom electrodes (notshown) since the third semiconductor region 703, which is generally thechannel region of the vertical power device 700, can be routed out ofthe plane of the drawing and contacted elsewhere separately from anycontact to either of the other two semiconductor regions 701 and/or 702.(These options for contacting the second and third semiconductor regions702 and 703 generally do not affect the size of the overall IC chip.)The first semiconductor region 701, on the other hand, is preferablyconnected through a bottom electrode 708, since the gate region 704 andthe oxide/insulator 705 obscure the front side of the firstsemiconductor region 701. Therefore, all three semiconductor regions701, 702 and 703 of the vertical power device 700 can potentially becontacted on the back side which provides significant advantages interms of heat dissipation as described above. In addition, thisconfiguration maintains the benefits described above regarding separatecontacts for the second semiconductor region 702 and the thirdsemiconductor region 703. Also, due to this configuration, the area ofthe second or third semiconductor region 702 or 703 that is availablefor contacting the top or bottom electrodes 706 or 707 (and similarelectrodes for the third semiconductor region 703) may be greater thanis conventional without significantly increasing the size of the overallIC chip. A larger contact size has the benefit of decreasing theresistance between the second or third semiconductor region 702 or 703and the top or bottom electrodes 706 or 707 (and similar electrodes forthe third semiconductor region 703).

Also shown in FIG. 7 is a trench region 709 (e.g. similar to trenchregion 310 or 410 of FIG. 3 or 4). The trench region 709 preferablyextends along an entire vertical side 710 of the second semiconductorregion 702 (and of the third semiconductor region 703, but outside ofthe plane of the drawing). Thus, the trench region 709 generallypenetrates through the entire active layer of the vertical power device700. Additionally, the trench region 709 generally horizontallysurrounds an entire active area of the vertical power device 700 (or themultiple fingers or the multiple power devices of which the verticalpower device 700 is a part). The active area thus surrounded isgenerally electrically isolated from other active areas of other powerdevices or transistors on the same die. The manufacturing or fabricationprocess (including thinning of the semiconductor substrate) generallyenables this feature for this and other embodiments of the presentinvention, as described below.

Some embodiments of the present invention can be described withreference to FIG. 8. FIG. 8 displays a top view of a layout pattern fora vertical power device 800 that may form part of an overall integratedcircuit (IC) chip and that is in accordance with the present invention.FIG. 8 will be described with reference to an n-type vertical powerdevice with a drain region connected on the back side. However, asimilar layout pattern will work for a p-type vertical power transistorand for a vertical power transistor having a drain region connected onthe top side. The vertical power device 800 generally comprises a gateelectrode 801 which is coupled to poly-silicon running along a gateregion 802, which includes gate edge regions 803 delineated by dashedlines. The part of the gate region 802 between the two dashed linescovers a strip of material that generally forms the first semiconductorregion 701 in FIG. 7, which in this case is the n-type drain of thevertical power device 800. The gate edge regions 803 cover two strips ofmaterial that generally form the third semiconductor region 703 in FIG.7, which in this case is the p-type channel of the vertical power device800. Source regions 804 generally comprise the second semiconductorregion 702 in FIG. 7, which in this case is the n-type source of thevertical power device 800. The source regions 804 preferably do notcover any other active material. Channel regions 805 are exposedportions of active semiconductor material that are coupled to thechannel region (the third semiconductor region 703) covered by the gateedge region 803. The channel regions 805 comprise the same material thatis covered by the poly-silicon of the gate-edge region 803 which in thiscase is the p-type channel of the vertical power device 800. The channelregions 805 are exposed to provide channel contacts and, in someembodiments, they can be made just large enough to allow for a singleelectrical contact to save space. This layout is generally more spaceefficient than the layout shown in FIG. 5, but the series resistance ofthe body voltage will be slightly higher since there are regions of thechannel (e.g. along the length of the source regions 804) that will berelatively far from the channel contacts. Additionally, the entirestructure of the vertical power device 800 is preferably horizontallysurrounded by the trench region 709 (FIG. 7) to electrically isolate thevertical power device 800 from other active areas of other power devicesor transistors on the same die.

Some embodiments of the present invention can be described withreference to FIG. 9. FIG. 9 displays a top view of a layout pattern fora vertical power device 900 that may form part of an overall integratedcircuit (IC) chip and that is in accordance with the present invention.FIG. 9 will be described with reference to an n-type vertical powerdevice with a drain region connected on the back side. However, asimilar layout pattern will work for a p-type vertical power transistorand for a vertical power transistor having a drain region contacted onthe top side. The vertical power device 900 generally comprises gateregions 901, 902 and 903, which generally comprise a poly-siliconmaterial. The vertical power device 900 also generally comprises bodycontacts 904 which preferably allow for either top or bottom contact toa region of p-type material that forms a channel region of the verticalpower device 900 generally under the gate regions 901 and 903. Thevertical power device 900 also generally comprises an n-type materialthat forms the drain of the vertical power device 900 under the gateregion 902 and an n-type material that forms a source of the verticalpower device 900 in regions 905. The p-type material under the gateregion 903 generally isolates the drain under the gate region 902 fromthe source regions 905. The source regions 905 preferably allow foreither top or bottom contact. Additionally, the entire structure of thevertical power device 900 is preferably horizontally surrounded by thetrench region 709 (FIG. 7) to electrically isolate the vertical powerdevice 900 from other active areas of other power devices or transistorson the same die.

Some embodiments of the present invention can be described withreference to FIG. 10. FIG. 10 displays a vertical power device 1000 thatmay form part of an overall integrated circuit (IC) chip and that is inaccordance with the present invention. The vertical power device 1000generally comprises first, second and third semiconductor regions 1001,1002 and 1003 within an active surface layer. The vertical power device1000 also has a gate region 1004 (surrounded by an oxide/insulator 1005)over the active layer. The first semiconductor region 1001 and thesecond semiconductor region 1002 are generally isolated from each otherby the third semiconductor region 1003. In some embodiments of thepresent invention, the first semiconductor region 1001 preferably servesas the source of the vertical power device 1000 and the secondsemiconductor region 1002 preferably serves as the drain of the verticalpower device 1000. Similar to embodiments in accordance with FIGS. 3 and4, no portion of the first semiconductor region 1001 is below the thirdsemiconductor region 1003, i.e. vertical and horizontal dimensions ofthe first semiconductor region 1001 have been minimized (e.g. tominimize parasitic capacitance, thermal resistance and electricalresistance). One or more top electrodes 1006 preferably contact thoseregions (e.g. the second semiconductor region 1002) that it is desiredto contact from the top side. The back side of the vertical power device1000, however, is generally covered by a metal contact (bottom sideelectrode) 1007 which preferably comprises either a single solder bumpor a copper pillar. In some embodiments of the present invention, thissingle metal contact 1007 is preferably deposited upon an interveningdielectric layer (not shown) with portions removed to provide for acontact from the metal to the active layers (the first and thirdsemiconductor regions 1003 and 1001). In some embodiments of the presentinvention with very low thermal resistance, all of the interveningdielectric layers between the vertical power device 1000 and the metalcontact 1007 are preferably removed. In some embodiments of the presentinvention, a shared backside contact (e.g. similar to metal contact1007) can also be applied to embodiments of the present invention thatare in accordance with those taught by FIG. 7. In this case, the channelcontact (for the third semiconductor region 703) can be shared witheither of the other two semiconductor regions 701 or 702 depending uponwhether either of the semiconductor regions 701 or 702 are defined asthe source or drain. However, in these embodiments a single back sidecontact may have to be applied to a patterned back side insulating layeror be applied in a careful pattern to avoid shorting the source anddrain.

In some embodiments of the present invention, the metal contact 1007 ispreferably shared among multiple fingers of a single power transistor,of which the vertical power device 1000 comprises a part. Typical solderbump and copper pillar dimensions are about 80-200 μm in diameter. Atypical transistor finger dimension can be on the order of 1 μm.Therefore, a single metal contact is typically much too large for asingle transistor finger. However, power transistors usually have manyfingers of transistor width arranged in parallel to provide a lowimpedance current path. The overall dimension of a many-fingered powertransistor is often on the order of a solder bump or copper pillar. Themetal contact 1007 may thus cover many transistor fingers of a singlepower transistor. Since the source and channel regions of all thefingers of a power transistor can be all held at the same potential,this configuration generally presents no problem for the operation ofthe power transistor. As mentioned previously, in some embodiments ofthe present invention having many-fingered power transistors, all of theintervening dielectric can be removed between the first semiconductorregion 1001, the third semiconductor region 1003 and the metal contact1007.

Several benefits accrue to embodiments of the present invention that arein accordance with the principles taught by FIG. 10. For example, thevertical power device 1000 can be extremely space efficient, because asingle contact can be used for the body and source of the vertical powerdevice 1000 as in the top-contacted vertical power device 300 of FIG. 3.However, the metal contact 1007 generally provides a significant thermalperformance enhancement for the vertical power device 1000, because themetal contact 1007 provides a direct thermal path out of the overall ICchip, and because nearly the entire area of active material for both thesource and body regions (i.e. the semiconductor regions 1001 or 1003) isin direct contact with this efficient direct thermal path. Furthermore,because the semiconductor layer is generally much thinner than that ofthe prior art, the thermal resistance vertically through thesemiconductor layer from the heat-generating active layer to the metalcontact 1007 is extremely low, resulting in extremely efficient heatremoval from the vertical power device 1000.

Also shown in FIG. 10 is a trench region 1008 (e.g. similar to trenchregion 310, 410 or 709 of FIG. 3, 4 or 7). The trench region 1008preferably extends along an entire vertical side 1009 of the thirdsemiconductor region 1003. Thus, the trench region 1008 generallypenetrates through the entire active layer of the vertical power device1000. Additionally, the trench region 1008 generally horizontallysurrounds an entire active area of the vertical power device 1000 (orthe multiple fingers or the multiple power devices of which the verticalpower device 1000 is a part). The active area thus surrounded isgenerally electrically isolated from other active areas of other powerdevices or transistors on the same die. Additionally, the metal contact1007 may be relatively large, generally extending throughout an areadefined or enclosed by the trench region 1008, within which are multiplefingers or multiple power devices of which the vertical power device1000 is a part. Furthermore, multiple such areas defined or enclosed bymultiple trench regions 1008 may be in a single IC chip, and each sucharea may have a separate relatively large metal contact 1007. Themanufacturing or fabrication process (including thinning of thesemiconductor substrate) generally enables this feature for this andother embodiments of the present invention, as described below.

Some embodiments of the present invention can be described withreference to FIG. 11. FIG. 11 displays a vertical Insulated Gate BipolarTransistor (IGBT) device 1100 that may form part of an overallintegrated circuit (IC) chip and that is in accordance with the presentinvention. An IGBT device is very similar to a vertical power device andcan be manufactured using very similar techniques to those described forthe vertical power devices described herein. The IGBT device 1100generally comprises a base region 1101, a channel region 1102, anemitter region 1103 and a collector region 1104 within an active surfacelayer. The IGBT device 1100 also generally comprises a gate region 1105,an emitter/base electrode 1106 and a bottom side collector electrode1107. Due to the manufacturing technique described below (includingthinning of the semiconductor substrate and further backsideprocessing), the collector electrode 1107 is preferably placed on theback side of the overall IC chip. Also, the IGBT device 1100 ispreferably thinned from top to bottom, resulting in reduced verticalresistance. Additionally, similar to the removal of a portion of thefirst semiconductor region 301 from beneath the third semiconductorregion 303 in FIG. 3, a portion of the channel region 1102 is preferablyremoved from beneath a bottom boundary 1108 of the base region 1101,i.e. vertical and horizontal dimensions of the channel region 1102 havebeen minimized (e.g. to minimize parasitic capacitance, thermalresistance and electrical resistance). In general, though, the channelregion 1102 must be left with a minimum thickness between the baseregion 1101 and the collector region 1104, so the base region 1101 andthe collector region 1104 do not short out or break down duringoperation. Furthermore, the overall thinning of the IGBT device 1100generally results in lower thermal resistance.

Also shown in FIG. 11 is a trench region 1109 (e.g. similar to trenchregion 310, 410, 709 or 1008 of FIG. 3, 4, 7 or 10). The trench region1109 preferably extends along an entire vertical side 1110 of the baseregion 1101. Thus, the trench region 1109 generally penetrates throughthe entire active layer of the IGBT device 1100. Additionally, thetrench region 1109 generally horizontally surrounds an entire activearea of the IGBT device 1100 (or the multiple fingers or the multipledevices of which the IGBT device 1100 is a part). The active area thussurrounded is generally electrically isolated from other active areas ofother devices or transistors on the same die. The manufacturing orfabrication process (including thinning of the semiconductor substrate)generally enables this feature for this and other embodiments of thepresent invention, as described below.

Some embodiments of the present invention can be described withreference to FIG. 12. FIG. 12 displays a vertical bipolar transistor1200 that may form part of an overall integrated circuit (IC) chip andthat is in accordance with the present invention. The vertical bipolartransistor 1200 generally comprises an emitter region 1201, a baseregion 1202 and a collector region 1203 within an active surface layerof a substrate in a vertical orientation. The vertical bipolartransistor 1200 also generally comprises an emitter electrode 1204, abase electrode 1205 and a bottom side collector electrode 1206. Due tothe manufacturing technique described below (including thinning of thesemiconductor substrate and further backside processing), the collectorelectrode 1206 is preferably placed on the back side of the overall ICchip. Additionally, the collector region 1203 is optionally formed byback side implantation/doping. As a result, this configuration generallyeliminates a lateral buried layer or a vertical sinker region (common inprior art devices) and the resistance due to these structures.Additionally, this configuration generally results in greater junctionisolation, thereby generally eliminating or minimizing parasiticcapacitance of the collector region 1203 to the substrate. Furthermore,an NPNP latch-up path (common in prior art devices) is also eliminated.In addition, the overall thinning of the vertical bipolar transistor1200 generally results in lower thermal resistance.

Also shown in FIG. 12 is a trench region 1207 (e.g. similar to trenchregion 310, 410, 709, 1008 or 11081109 of FIG. 3, 4, 7, 10 or 11). Thetrench region 1207 preferably extends along an entire vertical side 1208of the base region 1202 and the collector region 1203 and/or anyremaining substrate. Thus, the trench region 1207 generally penetratesthrough the entire active layer of the vertical bipolar transistor 1200.Additionally, the trench region 1207 generally horizontally surrounds anentire active area of the vertical bipolar transistor 1200 (or themultiple fingers or the multiple devices of which the vertical bipolartransistor 1200 is a part). The active area thus surrounded is generallyelectrically isolated from other active areas of other devices ortransistors on the same die. The manufacturing or fabrication process(including thinning of the semiconductor substrate) generally enablesthis feature for this and other embodiments of the present invention, asdescribed below.

Some embodiments of the present invention can be described withreference to FIG. 13. FIG. 13 displays a vertical UMOS (or trench-MOS)device 1300 that may form part of an overall integrated circuit (IC)chip and that is in accordance with the present invention. The UMOSdevice 1300 generally comprises a first semiconductor (e.g. a drain)region 1301, a second semiconductor (e.g. a source) region 1302, a thirdsemiconductor (e.g. a body/channel) region 1303 and a gate region 1304(surrounded by an oxide/insulator 1305) within an active surface layer.The UMOS device 1300 also generally comprises a top electrode 1306 and abottom electrode 1307. The manufacturing technique described below(including thinning of the semiconductor substrate and further backsideprocessing), generally enables elimination of a (typically) n− regionbetween the bottom of the gate region 1304 and the first semiconductor(e.g. the drain) region 1301. Therefore, the gate region 1304 preferablyextends as a trench to the first semiconductor region 1301 or to theback side of the silicon substrate, resulting in a form of trenchisolation and a lack of field concentration at the bottom of the trench.Consequently, the smooth U-shape of the bottom of the gate region oftypical prior art UMOS devices is generally no longer required orbeneficial. Additionally, similar to the above described vertical powerdevices, the silicon substrate is generally thinned, thereby reducingthe vertical resistance of the UMOS device 1300 and allowing for greaterthermal dissipation.

Also shown in FIG. 13 is a trench region 1308 (e.g. similar to trenchregion 310, 410, 709, 1008, 11081109 or 1207 of FIG. 3, 4, 7, 10, 11 or12). The trench region 1308 preferably extends along an entire verticalside 1309 of the first and third semiconductor regions 1301 and 1303.Thus, the trench region 1308 generally penetrates through the entireactive layer of the UMOS device 1300. Additionally, the trench region1308 generally horizontally surrounds an entire active area of the UMOSdevice 1300 (or the multiple fingers or the multiple devices of whichthe UMOS device 1300 is a part). The active area thus surrounded isgenerally electrically isolated from other active areas of other devicesor transistors on the same die. The manufacturing or fabrication process(including thinning of the semiconductor substrate) generally enablesthis feature for this and other embodiments of the present invention, asdescribed below.

Some embodiments of the present invention can be described withreference to FIG. 14. FIG. 14 displays an alternative vertical UMOS (ortrench-MOS) device 1400 that may form part of an overall integratedcircuit (IC) chip and that is in accordance with the present invention.The UMOS device 1400 generally comprises a first semiconductor (e.g. asource) region 1401, a second semiconductor (e.g. a drain) region 1402,a third semiconductor (e.g. a body/channel) region 1403 and a gateregion 1404 (surrounded by an oxide/insulator 1405) within an activesurface layer. The UMOS device 1400 also generally comprises a topelectrode 1406 and a bottom electrode (e.g. a contact, bump, pillar,etc.) 1407. In this embodiment, the gate region 1404 is so deep and thesilicon substrate is so thinned that the gate region 1404 extends to theback side of the silicon substrate, resulting in a form of trenchisolation and a lack of field concentration at the bottom of the trench.The large bottom electrode 1407 at the backside generally shorts out thefirst and third semiconductor regions (i.e. source and channel) 1401 and1403, leaving the drain contact (i.e. the top electrode 1406) at thetop. (This configuration thus has some similarities to the verticalpower device 1000 described above with the bottom source/channel metalcontact 1007.) The thinned silicon substrate reduces the verticalelectrical resistance and thermal resistance. The large bottom electrode1407 enhances the thermal dissipation.

Also shown in FIG. 14 is a trench region 1408 (e.g. similar to trenchregion 310, 410, 709, 1008, 11081109, 1207 or 1308 of FIG. 3, 4, 7, 10,11, 12 or 13). The trench region 1408 preferably extends along an entirevertical side 1409 of the third semiconductor region 1403. Thus, thetrench region 1408 generally penetrates through the entire active layerof the UMOS device 1400. Additionally, the trench region 1408 generallyhorizontally surrounds an entire active area of the UMOS device 1400 (orthe multiple fingers or the multiple devices of which the UMOS device1400 is a part). The active area thus surrounded is generallyelectrically isolated from other active areas of other devices ortransistors on the same die. The manufacturing or fabrication process(including thinning of the semiconductor substrate) generally enablesthis feature for this and other embodiments of the present invention, asdescribed below.

Some embodiments of the present invention can be described withreference to FIG. 15. FIG. 15 displays a vertical Gate Turn Off (GTO)thyristor 1500 that may form part of an overall integrated circuit (IC)chip and that is in accordance with the present invention. (A GTOthyristor is generally a controllable switch that can be turned on andoff at a gate.) The GTO thyristor 1500 generally comprises a gate region1501, a cathode region 1502, an anode region 1503, an N− region 1504 anda P region 1505 within an active surface layer. The GTO thyristor 1500also generally comprises a gate contact 1506, a cathode contact 1507 anda bottom side anode contact 1508. (The GTO thyristor 1500 in thisembodiment has a PN-PN structure from the anode region 1503 to thecathode region 1502. Other structure configurations are possible andwithin the scope of the present invention.) In general, the thinning ofthe silicon substrate, mention above for other embodiments, enables theGTO thyristor 1500 to be manufactured in the vertical configurationshown with low vertical electrical resistance and low thermalresistance. Additionally, the N− region 1504 can be made as thin asdesired (e.g. about 0.1 μm to 1 μm in thickness) for high-performance,low-voltage operation.

Also shown in FIG. 15 is a trench region 1509 (e.g. similar to trenchregion 310, 410, 709, 1008, 11081109, 1207, 1308 or 1408 of FIG. 3, 4,7, 10, 11, 12, 13 or 14). The trench region 1509 preferably extendsalong an entire vertical side 1510 of the regions 1503, 1504 and 1505.Thus, the trench region 1509 generally penetrates through the entireactive layer of the GTO thyristor 1500. Additionally, the trench region1509 generally horizontally surrounds an entire active area of the GTOthyristor 1500 (or the multiple fingers or the multiple devices of whichthe GTO thyristor 1500 is a part). The active area thus surrounded isgenerally electrically isolated from other active areas of other devicesor transistors on the same die. The manufacturing or fabrication process(including thinning of the semiconductor substrate) generally enablesthis feature for this and other embodiments of the present invention, asdescribed below.

Some embodiments of the present invention can be described withreference to FIGS. 16 a-b. FIG. 16 a displays a layer transfer device1600 that may form part of an overall integrated circuit (IC) chip andthat is in accordance with the present invention. The layer transferdevice 1600 is generally a vertical power device with all transistornodes contacted through the back side. The layer transfer device 1600generally comprises a handle wafer layer 1601 and an initial wafer (e.g.formed with SOI or bulk semiconductor) layer 1602. The handle waferlayer 1601 generally comprises a handle substrate layer 1603 and ahandle bond layer 1604. The initial wafer layer 1602 generally comprisesan active layer 1605, an insulator layer 1606, a gate region 1607(surrounded by an oxide/insulator 1608) and conductive (e.g. metal)wiring 1609 (separated by insulators). The active layer 1605 generallycomprises first, second and third semiconductor regions 1610, 1611 and1612. A semiconductor substrate in which the first, second and thirdsemiconductor regions 1610, 1611 and 1612 are formed has preferablyundergone thinning, as mentioned above for other embodiments. Similar toembodiments in accordance with FIGS. 3, 4 and 10, no portion of thefirst semiconductor region 1610 is below the third semiconductor region1612, i.e. vertical and horizontal dimensions of the first semiconductorregion 1610 have been minimized (e.g. to minimize parasitic capacitance,thermal resistance and electrical resistance). In the illustratedembodiment, the first semiconductor region 1610 and the thirdsemiconductor region 1612 are both contacted to a single back side metalcontact (or bottom electrode) 1613. Similar to the embodimentillustrated in FIG. 10, the third semiconductor region 1612 ispreferably the body of the layer transfer vertical power device 1600,and the first semiconductor region 1610 is preferably the source. Thesecond semiconductor region 1611 is preferably the drain of the layertransfer vertical power device 1600. The gate region 1607 and top sidedrain electrodes 1614 are preferably contacted through the metal wiring1609 routed in the active layer 1605 through the back side of the layertransfer vertical power device 1600 to contacts 1615.

Several benefits accrue to the use of backside-contacted layer transfervertical power devices in accordance with FIGS. 16 a-b. The use ofsolder bumps or copper pillars on backside contacted layer transferdevices provides very low electrical impedance to ground. There areseveral reasons for this. One is that the distance through a bump orpillar is much shorter than the distance of a bond wire plus packagelead. The shorter distance reduces both the resistance and theinductance of the transistor connections. Also, low source impedanceimproves efficiency of power devices. Additionally, flip-chip processingalso can improve the isolation between connections of an integratedpower device because bond wire and package interactions are avoided.Furthermore, since bumps or pillars can be placed at both the perimeterand across the surface of a die, while bond wire pads can only be placedat the perimeter of a die, a bumped or pillared die can have shorterlateral paths from active devices to off-chip electrical connections.Since the on-chip metallization can become the dominant seriesresistance of a large power device, any reduction in lateralmetallization resistance will result in improved power deviceefficiency. The placement of the bumps or pillars directly over orcloser to a transistor at the center of a die generally results inreduced lateral metallization resistance.

Also shown in FIG. 16 a is a trench region 1616 (e.g. similar to trenchregion 310, 410, 709, 1008, 11081109, 1207, 1308, 1408 or 1509 of FIG.3, 4, 7 or 10-15). The trench region 1616 preferably extends along anentire vertical side 1617 of the third semiconductor region 1612. Thus,the trench region 1616 generally penetrates through the entire activelayer 1605 of the vertical power device 1600. Additionally, the trenchregion 1616 generally horizontally surrounds an entire active area ofthe layer transfer vertical power device 1600 (or the multiple fingersor the multiple power devices of which the layer transfer vertical powerdevice 1600 is a part). The active area thus surrounded is generallyelectrically isolated from other active areas of other power devices ortransistors on the same die. The manufacturing or fabrication process(including thinning of the semiconductor substrate) generally enablesthis feature for this and other embodiments of the present invention, asdescribed below.

Benefits accrue to the use of the trench regions 1616 in accordance withFIGS. 16 a-b. The presence of the trench regions 1616 within layertransfer vertical power devices, for instance, provides complete (oralmost complete) dielectric isolation between the transistors in anoverall integrated power device. Therefore, almost any circuitconfiguration can be obtained with the power transistors. Withoutisolation, on the other hand, only common drain circuits orsingle-transistor devices can be made with VDMOS. Additionally,conductive substrate noise can be completely (or almost completely)eliminated or minimized with dielectric isolation of the transistors.While some capacitive coupling will still be present, the transistorscan be dielectrically isolated where desired. Substrate noise is a verylarge practical problem in integrated power devices that have largeswitching transistors that generate large amounts of substrate noisewhich can be picked up by sensitive analog circuits or can even causedigital circuit malfunction. Since so many alternate paths are typicallypresent in bulk integrated power devices, except as described herein, itcan be very difficult to isolate the specific causes and remedies forsubstrate-noise-induced problems.

FIG. 16 b shows the embodiment of FIG. 16 a with an added layer ofpost-layer-transfer metallization. In this embodiment, contacts 1620 arecut through the insulator layer 1606. A back-side interconnect metallayer 1619 is formed on the back surface of the insulator layer 1606.This interconnect metal extends through the contacts 1620 or connects toa contact-filling conductor. A passivation layer 1618 is formedoverlying the interconnect layer 1619, and electrodes 1613 and 1615 areformed through openings in the passivation layer so as to connect to theinterconnect layer 1619. The interconnect layer 1619 may allow, forexample, the back-side electrodes 1613 and 1615 to be positioned inareas remote from the layer transfer device 1600, rather than directlybehind it. The interconnect layer 1619 may also allow the layer transferdevice 1600 to be electrically connected to other devices in theintegrated circuit.

Although the embodiments described above with reference to FIGS. 3, 4, 7and 10-15 do not show a layer transfer device, it is understood thatthese embodiments are not necessarily so limited. Rather, variations ofthese embodiments are compatible with a layer transfer device, similarto the layer transfer vertical power device 1600. Therefore, many, ifnot all, of the benefits that thus accrue to the layer transfer verticalpower device 1600, by virtue of being manufactured with layer transfertechniques, may also apply to these other embodiments.

Some embodiments of the present invention can be described withreference to FIG. 17. FIG. 17 displays a silicon die (or IC chip) 1700that is in accordance with the present invention. The silicon die 1700generally comprises a first region 1701 having a first thickness and asecond region 1702 having a second thickness smaller than the thicknessof the first region 1701. The silicon in the first region 1701, forexample, could have a thickness of about 0.8 μm and could comprise oneor more vertical devices 1703, as described above. The silicon in thesecond region 1702, for example, could have a thickness of 80 nanometers(nm) and could comprise various high-performance switches and low-powerdigital logic (i.e. non-vertical semiconductor devices) 1704. Anydesired number and combination of the vertical semiconductor devicesdescribed above can be combined on a single die in this fashion withinthe first region 1701. Some embodiments of the present invention couldtherefore allow for optimized power devices alongside other functions,such as RF switching or digital logic blocks. In some embodiments of thepresent invention, regions of trench isolation 1705 (e.g. similar totrench region 310, 410, 709, 1008, 11081109, 1207, 1308, 1408, 1509 or1616 of FIG. 3, 4, 7 or 10-16) penetrating through entire active layerswithin the first and second regions 1701 down to a buried insulatorlayer (not shown) preferably provide sufficient electrical isolation forthe different portions of the die 1700 to operate independently of eachother. As a result, not only can the silicon die 1700 have multiplevertical semiconductor devices, but the bottom side of each device canbe accessed and contacted independently of the others. It is, therefore,unnecessary for the multiple devices to have a common drain, forexample. Additionally, such embodiments enable a vertical power device(such as those described above) to be integrated on a single IC chip orsilicon die with various converters, analog circuitry and amicroprocessor (among other possible components) and yet be able toefficiently dissipate heat (e.g. through the back side) generated by thevarious components.

In some embodiments of the present invention, the doping of the firstsemiconductor region (as described above) will be much higher near theback side contact area. This configuration allows for reduced resistancein the first semiconductor area and lower back side contact resistance.In some embodiments wherein the first semiconductor region is the drainregion of the vertical power device, this configuration will result inlower drain resistance while keeping the threshold voltage low andsaturation drive current high. In some embodiments of the presentinvention, the gate insulator (as described above) can be thicker in thecenter of the drain region. This will result in a reduced gate to draincapacitance. Embodiments of the present invention that are in accordancewith these principles will generally exhibit improved speed performancegiven the reduction of internal capacitance and resistance.

FIG. 18 shows a process 1800 for fabricating one or more of the devices(e.g. similar to device 300, 400, 500, 600, 700, 800, 900, 1000, 1300,1400, 1600, 1703 or 1704) shown in FIGS. 3-10 and 13, 14, 16 and 17,according to some embodiments of the present invention. It isunderstood, however, that the specific process 1800 is shown forillustrative purposes only and that other embodiments (in addition tospecifically mentioned alternative embodiments) may involve otherprocesses or multiple processes with other individual steps or adifferent order or combination of steps and still be within the scope ofthe present invention.

The process 1800 preferably starts (at 1801) with an SOI wafer or a bulksemiconductor wafer. An acceptable SOI wafer for some of the abovedescribed embodiments preferably has a top silicon (Si) layer, e.g.about 0.2-1.0 μm to tens of microns in thickness.

At 1802, various trench isolation regions (e.g. similar to trench region310, 410, 709, 1008, 1308, 1408, 1616 or 1705) are preferably patterned,etched and deposited/filled to isolate (as desired) the various devicesthat are to be formed in the wafer. The trench isolation regions may beformed by a trench etch or by a through-semiconductor via (TSV) etch toform relatively deep trenches or TSV structures followed by placement ofan oxide/insulating material, as desired. For embodiments using an SOIwafer, the trench isolation regions are preferably formed down to (oralmost down to, or at least down to) the buried oxide layer.Additionally, the vertical gate trench (including gate polysilicon) forthe gate region 1304 or 1404 and the oxide/insulator 1305 or 1405 of theUMOS device 1300 or 1400 may be formed with the various trench isolationregions or formed in one or more separate processing steps as desired.Additionally, the trench isolation regions are preferably formed deepenough into the wafer that subsequent thinning of the wafer or removalof bottom portions of the wafer will result in the trench isolationregions generally penetrating through the entire active layer of theremaining wafer.

At 1803, for embodiments according to FIG. 17, ahigh-temperature-tolerant epitaxial masking layer such as SiO2 or Si3N4is preferably patterned on the wafer. Then silicon is preferablyepitaxially deposited in region 1701 in sufficient amount for thevertical device(s) that are to be formed. Alternatively, the top siliconof the wafer is patterned and removed (or thinned) preferentially forthe second region 1702. This removal can be done, for example, with asilicon etch, with preferential consumption of silicon through oxidationin a LOCOS-style process step. The silicon may be thinned as desired tomake SOI CMOS devices (for example, if fully-depleted CMOS devices aredesired for high-performance RF switch applications).

At 1804, the central drain or source region is patterned and implanted,e.g. with N− dopant in region 301, 401, 701, 1001, 1610. For embodimentsaccording to FIGS. 13 and 14 (UMOS examples), on the other hand, thelower drain or source region is formed as described below.

At 1805, channel region doping is patterned and implanted, e.g. inregion 303, 403, 506, 604, 703, 805, 904, 1003, 1303, 1403 or 1612. Forembodiments according to FIG. 17, the channel doping may be implantedfor both the first and second regions 1701 and 1702 at 1805 or inseparate fabrication steps. Alternatively, 1805 may be skipped ifappropriate channel doping is present in the top silicon layer when theSOI wafer or the bulk semiconductor wafer is manufactured. Optionally,the channel doping may be implanted later, as described below.

At 1806, deep drain or source region doping is patterned and implanted,e.g. with N+ dopant in region 301, 401, 701, 1001, 1301, 1401 or 1610.For embodiments according to FIG. 17, the deep drain or source doping ispreferably performed for the first region 1701. Optionally, since theregions 301, 401, 701, 1001, 1301, 1401 and 1610 are accessible from theback side of the wafer after further processing, 1806 may be skipped,and the appropriate doping may be done through the back side later, asdescribed below.

At 1807, gate polysilicon is deposited, doped and patterned, e.g. inregion 304, 404, 503, 504, 601, 602, 704, 801, 802, 901, 902, 903, 1004or 1607. For embodiments according to FIGS. 13 and 14 (UMOS examples),on the other hand, the gate polysilicon was preferably formed asdescribed above.

At 1808, channel contact areas are patterned and shallow and/or deepimplanted, e.g. with P+ dopant in exposed portions of region 303, 403,506, 604, 703, 805, 904, 1003, 1303, 1403 or 1612. For embodimentsaccording to FIG. 17, the channel contact and implantation is preferablyperformed for the first region 1701. Optionally, since the regions 403,506, 604, 703, 805, 904, 1003, 1403 or 1612 are accessible from the backside of the wafer after further processing, 1808 may be skipped, and theappropriate patterning and implanting may be done through the back sidelater, as described below. Alternatively, this patterning and implantingmay be done on both the front and back sides.

At 1809, source or drain region doping is patterned and shallow and/ordeep implanted, e.g. with N+ dopant in region 302, 402, 505, 603, 702,804, 905, 1002, 1302, 1402 or 1611. For embodiments according to FIG.17, the source or drain doping is preferably performed in both the firstand second regions 1701 and 1702, but optionally each region 1701 and1702 can be separately patterned and implanted.

At 1810, contact and metallization layers (with separating dielectricmaterial) are formed for top side connections (e.g. top electrodes 308,407, 706, 1006, 1306, 1406 and 1614, the metal wiring 1609, etc.) to thegate region, the source or drain regions and/or the channel regions (asdesired and if accessible from the top side), e.g. for region 302, 303,304, 402, 403, 404, 503, 505, 506, 601, 603, 604, 702, 703, 704, 801,804, 805, 901, 902, 904, 905, 1002, 1004, 1302, 1303, 1304, 1402, 1404,1607 or 1611. For embodiments according to FIG. 17, the gate, source,drain and/or channel contacts and metallization may generally be madefor both the first and second regions 1701 and 1702 as desired.

At 1811, a handle wafer (e.g. for embodiments using the handle waferlayer 1601) is preferably bonded to the exposed top surface of theoriginal wafer that has generally received the preceding processing,e.g. the SOI wafer. The handle wafer may be made of Si, quartz,sapphire, AlN, SiC, etc. Additionally, a heat spreading layer mayoptionally be placed between the SOI wafer and the handle wafer. Forembodiments not using the handle wafer layer 1601, the handle wafer maybe temporarily bonded to the original wafer if the handle wafer isneeded for physical support of the original wafer during subsequentprocessing. In which case, the handle wafer may be removed whenappropriate. For some embodiments (e.g. some embodiments using the bulksemiconductor wafer), it may not be necessary to perform 1811 if theoriginal wafer can be thinned or further processed, as described below,without needing the additional physical support of the handle wafer.

At 1812, an underlying portion of the original wafer is preferablyremoved or thinned. For embodiments using an SOI wafer, for example, thesubstrate under the buried oxide is generally removed up to (andpreferably including portions of) the buried oxide. For embodimentsusing a bulk semiconductor wafer, on the other hand, the substrate isgenerally thinned from the bottom side until the trench regions or TSVstructures are exposed. In this manner, the trench isolation regionsformed at 1802 are generally left penetrating through the entire activelayer of the remaining wafer, preferably with only an insulation layer(e.g. the buried oxide or a deposited insulator layer), if anything,underlying the active layer and the trench isolation regions at thispoint.

At 1813, the drain (or source) region is preferably selectively dopedfrom the back side (if not done through deep implantation above at1806), e.g. with N+ dopant in region 301, 401, 701, 1001, 1301, 1401 or1610. For embodiments according to FIG. 17, the back side drain orsource doping is preferably performed for the first region 1701.Additionally, if metals are present from the previous processing thatare tolerant only to low temperature processing, then the backsidedoping at 1813 can preferably be performed with an implant (whichtypically involves a low temperature) followed by dopant activation witha very short-time anneal, such as a laser or e-beam anneal.

At 1814, contact areas for the channel region (in embodiments in whichthe channel region is to be contacted at the back side) are preferablydoped from the back side, e.g. with P+ dopant in region 403, 703, 1003,1403, 1612. In some embodiments, such as the embodiment of FIG. 7, theback side contact areas for the channel region (e.g. 703) are outside ofthe plane of the drawing. For embodiments according to FIG. 17, the backside channel contact region is preferably doped for the first region1701.

At 1815, patterned contact and metallization are formed for desired backside connections (e.g. bottom electrodes and contacts 309, 408, 409,707, 708, 1007, 1307, 1407, 1613, etc.) to the drain regions, the sourceregions and/or the channel regions (as desired and if accessible fromthe bottom side), e.g. for region 301, 401, 403, 701, 702, 703, 1001,1003, 1301, 1401, 1403, 1610 or 1612. Further metallization (withseparating dielectric material) is also preferably performed to the backside for those regions having top side contacts that are routed down tothe back side, e.g. for contacts 1615. For embodiments according to FIG.17, the back side contacts are generally formed for both the first andsecond regions 1701 and 1702, as desired.

At 1816, various passivation deposition techniques are performed and padopenings are formed to generally complete the overall IC chip. Theprocess 1800 then preferably ends at 1817.

FIG. 19 shows a process 1900 for fabricating one or more of the devices(e.g. similar to the IGBT device 1100 or the vertical bipolar transistor1200) shown in FIGS. 11 and 12, according to some embodiments of thepresent invention. It is understood, however, that the specific process1900 is shown for illustrative purposes only and that other embodiments(in addition to specifically mentioned alternative embodiments) mayinvolve other processes or multiple processes with other individualsteps or a different order or combination of steps and still be withinthe scope of the present invention.

The process 1900 preferably starts (at 1901) with an SOI wafer or a bulksemiconductor wafer. An acceptable SOI wafer for some of the abovedescribed embodiments preferably has a top silicon (Si) layer, e.g.about 1.0 μm to tens of microns in thickness.

At 1902, similar to 1802 above, various trench isolation regions (e.g.similar to trench region 1109 or 1207) are preferably patterned, etchedand deposited/filled to isolate (as desired) the various devices thatare to be formed in the wafer. The trench isolation regions may beformed by a trench etch or by a TSV etch to form relatively deeptrenches or TSV structures followed by placement of an oxide/insulatingmaterial, as desired. For embodiments using an SOI wafer, the trenchisolation regions are preferably formed down to (or almost down to, orat least down to) the buried oxide layer. Additionally, the trenchisolation regions are preferably formed deep enough into the wafer thatsubsequent thinning of the wafer or removal of bottom portions of thewafer will result in the trench isolation regions generally penetratingthrough the entire active layer of the remaining wafer.

At 1903, similar to 1803 above, for embodiments according to FIG. 17, ahigh-temperature-tolerant epitaxial masking layer such as SiO2 or Si3N4is preferably patterned on the wafer. Then silicon is preferablyepitaxially deposited in region 1701 in sufficient amount for thevertical device(s) that are to be formed. Alternatively, the top siliconof the wafer is patterned and removed (or thinned) preferentially forthe second region 1702. This removal can be done, for example, with asilicon etch, with preferential consumption of silicon through oxidationin a LOCOS-style process step. The silicon may be thinned as desired tomake SOI CMOS devices (for example, if fully-depleted CMOS devices aredesired for high-performance RF switch applications).

At 1904, the channel region 1102 (FIG. 11) or the base region 1202 (FIG.12) doping is patterned and implanted. Alternatively, 1904 may beskipped if appropriate channel or base doping is present in the topsilicon layer when the SOI wafer or the bulk semiconductor wafer ismanufactured.

At 1905, the collector region 1104 or 1203 is patterned and deepimplanted, e.g. with P+ dopant. Alternatively, the deep collector dopingat 1905 is skipped, and the collector doping is done later from the backside, as described below.

At 1906, for embodiments according to FIG. 11, gate polysilicon isdeposited, doped and patterned, e.g. in the gate region 1105.

At 1907, the base contact areas are preferably patterned and implanted,e.g. with P+ dopant in region 1101 or 1202.

At 1908, the emitter region 1103 or 1201 is preferably patterned andimplanted, e.g. with N+ dopant.

At 1909, contact and metallization layers (with separating dielectricmaterial) are formed for top side connections (as desired and ifaccessible from the top side). For embodiments according to FIG. 11,contact and metallization layers are formed for the gate region 1105 andthe base and emitter regions 1101 and 1103 (e.g. the emitter/baseelectrode 1106). For embodiments according to FIG. 12, on the otherhand, contact and metallization layers are formed for the base region1202 (e.g. base electrode 1205) and the emitter region 1201 (e.g.emitter electrode 1204).

At 1910, similar to 1811 above, a handle wafer (e.g. for embodimentsusing the handle wafer layer 1601) is preferably bonded to the exposedtop surface of the original wafer that has generally received thepreceding processing, e.g. the SOI wafer. The handle wafer may be madeof Si, quartz, sapphire, AlN, SiC, etc. Additionally, a heat spreadinglayer may optionally be placed between the SOI wafer and the handlewafer. For embodiments not using the handle wafer layer 1601, the handlewafer may be temporarily bonded to the original wafer if the handlewafer is needed for physical support of the original wafer duringsubsequent processing. In which case, the handle wafer may be removedwhen appropriate. For some embodiments (e.g. some embodiments using thebulk semiconductor wafer), it may not be necessary to perform 1910 ifthe original wafer can be thinned or further processed, as describedbelow, without needing the additional physical support of the handlewafer.

At 1911, similar to 1812 above, an underlying portion of the originalwafer is preferably removed or thinned. For embodiments using an SOIwafer, for example, the substrate under the buried oxide is generallyremoved up to (and preferably including portions of) the buried oxide.For embodiments using a bulk semiconductor wafer, on the other hand, thesubstrate is generally thinned from the bottom side until the trenchregions or TSV structures are exposed. In this manner, the trenchisolation regions formed at 1902 are generally left penetrating throughthe entire active layer of the remaining wafer, preferably with only aninsulation layer (e.g. the buried oxide or a deposited insulator layer),if anything, underlying the active layer and the trench isolationregions at this point.

At 1912, the collector region 1104 or 1203 is preferably doped from theback side, if not already done by deep implantation from the front sideat 1905. For embodiments in accordance with FIG. 11, a P+ dopant isused. For embodiments in accordance with FIG. 12, on the other hand, anN+ dopant is used.

At 1913, similar to 1815 above, patterned contact and metallization areformed for desired back side connections (e.g. bottom electrodes andcontacts 1107, 1206, etc.) to the collector region 1104 or 1203. Furthermetallization (with separating dielectric material) is also preferablyperformed to the back side for those regions having top side contactsthat are routed down to the back side.

At 1914, similar to 1816 above, various passivation depositiontechniques are performed and pad openings are formed to generallycomplete the overall IC chip. The process 1900 then preferably ends at1915.

FIG. 20 shows a process 2000 for fabricating the device (e.g. similar todevice 1500) shown in FIG. 15, according to some embodiments of thepresent invention. It is understood, however, that the specific process2000 is shown for illustrative purposes only and that other embodiments(in addition to specifically mentioned alternative embodiments) mayinvolve other processes or multiple processes with other individualsteps or a different order or combination of steps and still be withinthe scope of the present invention.

The process 2000 preferably starts (at 2001) with an SOI wafer or a bulksemiconductor wafer. An acceptable SOI wafer for some of the abovedescribed embodiments preferably has a top silicon (Si) layer, e.g.about 1.0 μm to tens of microns in thickness.

At 2002, similar to 1802 or 1902 above, various trench isolation regions(e.g. similar to trench region 1509) are preferably patterned, etchedand deposited/filled to isolate (as desired) the various devices thatare to be formed in the wafer. The trench isolation regions may beformed by a trench etch or by a TSV etch to form relatively deeptrenches or TSV structures followed by placement of an oxide/insulatingmaterial, as desired. For embodiments using an SOI wafer, the trenchisolation regions are preferably formed down to (or almost down to, orat least down to) the buried oxide layer. Additionally, the trenchisolation regions are preferably formed deep enough into the wafer thatsubsequent thinning of the wafer or removal of bottom portions of thewafer will result in the trench isolation regions generally penetratingthrough the entire active layer of the remaining wafer.

At 2003, similar to 1803 or 1903 above, for embodiments according toFIG. 17, a high-temperature-tolerant epitaxial masking layer such asSiO2 or Si3N4 is preferably patterned on the wafer. Then silicon ispreferably epitaxially deposited in region 1701 in sufficient amount forthe vertical device(s) that are to be formed. Alternatively, the topsilicon of the wafer is patterned and removed (or thinned)preferentially for the second region 1702. This removal can be done, forexample, with a silicon etch, with preferential consumption of siliconthrough oxidation in a LOCOS-style process step. The silicon may bethinned as desired to make SOI CMOS devices (for example, iffully-depleted CMOS devices are desired for high-performance RF switchapplications).

At 2004, if N− doping is present in the top silicon layer when the SOIwafer or the bulk semiconductor wafer is manufactured, then the upper Pregion 1505 is preferably implanted with P dopant. Otherwise, 2004 ispreferably preceded by implanting the N− dopant in the N− region 1504.

At 2005, the anode region 1503 is preferably patterned and deepimplanted, e.g. with P+ dopant. Alternatively, the deep anode implantingat 2005 is skipped, and the anode implanting is done later from the backside, as described below.

At 2006, the gate region 1501 is preferably patterned and implanted,e.g. with P+ dopant. Additionally, at 2007, the cathode region 1502 ispreferably patterned and implanted, e.g. with N+ dopant.

At 2008, contact and metallization layers (with separating dielectricmaterial) are formed for top side connections, e.g. gate contact 1506and cathode contact 1507.

At 2009, similar to 1811 or 1910 above, a handle wafer (e.g. forembodiments using the handle wafer layer 1601) is preferably bonded tothe exposed top surface of the original wafer that has generallyreceived the preceding processing, e.g. the SOI wafer. The handle wafermay be made of Si, quartz, sapphire, AlN, SiC, etc. Additionally, a heatspreading layer may optionally be placed between the SOI wafer and thehandle wafer. For embodiments not using the handle wafer layer 1601, thehandle wafer may be temporarily bonded to the original wafer if thehandle wafer is needed for physical support of the original wafer duringsubsequent processing. In which case, the handle wafer may be removedwhen appropriate. For some embodiments (e.g. some embodiments using thebulk semiconductor wafer), it may not be necessary to perform 2009 ifthe original wafer can be thinned or further processed, as describedbelow, without needing the additional physical support of the handlewafer.

At 2010, similar to 1812 or 1911 above, an underlying portion of theoriginal wafer is preferably removed or thinned. For embodiments usingan SOI wafer, for example, the substrate under the buried oxide isgenerally removed up to (and preferably including portions of) theburied oxide. For embodiments using a bulk semiconductor wafer, on theother hand, the substrate is generally thinned from the bottom sideuntil the trench regions or TSV structures are exposed. In this manner,the trench isolation regions formed at 2002 are generally leftpenetrating through the entire active layer of the remaining wafer,preferably with only an insulation layer (e.g. the buried oxide or adeposited insulator layer), if anything, underlying the active layer andthe trench isolation regions at this point.

At 2011, the anode region 1503 is preferably doped from the back side,if not already done by deep implantation from the front side at 2005.

At 2012, similar to 1815 or 1913 above, patterned contact andmetallization are formed for desired back side connections (e.g. bottomside anode contact 1508, etc.) to the anode region 1503. Furthermetallization (with separating dielectric material) is also preferablyperformed to the back side for those regions having top side contactsthat are routed down to the back side.

At 2013, similar to 1816 or 1914 above, various passivation depositiontechniques are performed and pad openings are formed to generallycomplete the overall IC chip. The process 2000 then preferably ends at2014.

Although embodiments of the present invention have been discussedprimarily with respect to specific embodiments thereof, other variationsare possible. Various configurations of the described system may be usedin place of, or in addition to, the configurations presented herein. Forexample, multiple fingers of each type of power device discussed couldshare the same trench isolated area. Also, multiple types of powerdevices discussed herein could share the same trench isolated area,could share the same first semiconductor region, or could share both. Inaddition, the vertical power devices were often described using n-typedevices as an example but the present invention can implement p-type orn-type devices. Also, additional layers of passivation and insulationcould be disposed in-between described layers where appropriate.

FIG. 21 shows a flow chart 2100 describing an embodiment of the presentinvention wherein processing on the back side of the structure allowsfor improved device characteristics and more varied device types. Likethe flow chart 1800 shown in FIG. 18, flow chart 2100 begins, in step2110, by providing a semiconductor layer having a first surface and asecond surface, and doped to a first conductivity type. Thissemiconductor layer could be, for example, a bulk silicon wafer, or anSOI structure. The semiconductor layer could be doped, for example,uniformly n-type.

In step 2120 of FIG. 21, a first doped region is formed at the secondsurface of the semiconductor layer. This region is doped with a secondconductivity type, opposite the first conductivity type; for example,this first doped region is doped p-type. In step 2130, a second dopedregion of the first conductivity type (for example, n-type) is formed.The second doped region is also formed at the second surface of thesemiconductor layer, inside the first doped region. The doped regionsmay be formed, for example, by an ion implant step, followed by afurnace diffusion or rapid thermal anneal (RTA) step.

Other steps not shown in FIG. 21 may be performed, depending on thenature of the device desired. For example, before any doped regions areformed, isolation areas may be formed as described in step 1802 in FIG.18. Doped well regions (for CMOS devices) may also be formed. If avertical double-diffused MOS (DMOS) device is desired, a gate oxide isgrown and a gate polysilicon layer is deposited and patterned. The firstdoped region, which functions as the body region of the DMOS device, isthen formed (step 2120) followed by the second doped region (step 2130),which functions as the source of the vertical DMOS device. Alightly-doped (N-type) region may be formed at the edges of the gateprior to forming the (N-type) source region (step 2130), usingdielectric spacers on either side of the gate to separate the source andthe lightly-doped region.

In step 2140 of FIG. 21, an external electrical contact to the seconddoped region is formed. In the DMOS example, this would form the sourceelectrode of the device. This electrical contact could be formed, forexample, by forming a silicide at the surface of the second dopedregion; for example, titanium, cobalt, or tungsten silicide could beused. This contact formation step may also comprise forming a contacthole in a deposited dielectric and filling this hole with a metal.Forming one or more electrical wiring layers connecting to the filledcontacts may complete the electrical contact process. In certainembodiments, it may be desirable to use materials for these contactsthat can withstand later high-temperature processing. For example,refractory metals—for example, Tungsten—may be used for the contact filland wiring materials.

In step 2150 of FIG. 21, a handle layer is optionally coupled to thesecond surface of the semiconductor layer. Any of the materialsdiscussed in the description of step 1811 (FIG. 18) may be used. Forexample, a silicon wafer may be attached to the top surface of thedielectric and wiring layers formed in step 2140. In step 2160, aportion of the semiconductor layer is removed from the firstsurface—that is, the side opposite the doped regions—exposing a newthird surface of the semiconductor layer. If an SOI wafer is used, theburied oxide may be used as an etch stop for this step. The buried oxideexposed would then also be removed. If a bulk silicon wafer is used forthe semiconductor layer, a timed etch could be used; for example, atimed wet silicon etch using, for example, TMAH, KOH or HNO₃:HF could beused.

In step 2170 of FIG. 21, a third doped region is formed at the newlyexposed third surface of the semiconductor layer, on the back side ofthe device structure. For the DMOS example, this third doped regionwould be an N+ region. This layer could also be a P+ region if an IGBTis desired. In step 2180, a second electrical contact to the third dopedregion is formed on the third surface of the semiconductor layer (backside of the device structure). For the DMOS example, this electricalcontact functions as the drain contact of the device.

FIGS. 22 A-H illustrate an embodiment of the present invention describedby the flow chart 2100 in FIG. 21. Specifically, FIGS. 22 A-H show howprocessing on the back side of the semiconductor structure allows moreflexibility in device design. The process begins in FIG. 22A byproviding a semiconductor layer 220 having a first surface 223 and asecond surface 226. As in step 1801 of FIG. 18, the semiconductor layercould be a bulk semiconductor wafer, having a thickness of 50 to 1000 or300 to 800 or it could be an SOI wafer. The semiconductor layer 200 isdoped with a first conductivity type, for example, N-type. Thisbackground dopant may be, for example, Phosphorus. The background dopingof the semiconductor layer 200 may be, for example, spatially uniform.The doping concentration may be, for example, between 10¹⁴ and 10¹⁸cm⁻³, for example, between 10¹⁵ and 5×10¹⁷ cm-3, for example, between5×10¹⁶ and 2×10¹⁷ cm⁻³. The background doping level determines theblocking voltage of the device, in combination with the device's finalthickness.

In FIG. 22B, a first doped region 224 is formed at the second surface226 of the semiconductor layer 220. Isolation regions, through-siliconvias, and/or well regions may be formed prior to this step. Also shownin FIG. 22B are optional gate oxide regions 221 and gates 222. Thesefeatures are formed if gate-controlled vertical devices—e.g., verticalDMOS, IGBT, gate-turn-on (GTO) thyristors—are desired. The gate features222 may be formed, for example, by growing a thermal gate oxide,depositing a n-doped polysilicon layer, and patterning the polysiliconlayer and gate oxide stack. The first doped region 224 may be formedafter the gate patterning step, thus self-aligning these regions to thegate, or it may be performed prior to gate patterning using a separatemasking step. The doped regions 224 are doped with a dopant of oppositeconductivity type to the background dopant of the semiconductor layer220; for example, p-type. The peak concentration of the doped regions224 may be, for example, 10¹⁶ to 5×10¹⁸ cm⁻³; for example, 10¹⁷ to2×10¹⁸ cm⁻³. The doped regions 224 may be formed, for example, byimplanting boron into the surface 226 of the semiconductor layer 220, orby diffusing from a liquid or gaseous source such as BBr₃. In bothcases, this dopant introduction step may be followed by a furnace orrapid-thermal drive-in anneal. The first doped region 224 may functionas, for example, the body region of a DMOS device, the emitter/bodyregion of an IGBT, or the base region of a vertical bipolar or athyristor.

In FIG. 22C, a second doped region 225 is formed at the second surface226 of the semiconductor layer 220. The second doped region 225 is dopedto the same conductivity type as the background doping of thesemiconductor layer 220; for example, n-type. The second doped region225 is formed completely inside the first doped region 224. The peakconcentration of the second doped region 225 may be, for example, 5×10¹⁸to 10²¹ cm⁻³, for example, 10¹⁹ to 2×10²⁹ cm⁻³. The doped regions 225may be formed, for example, by implanting phosphorous or arsenic intothe surface 226 of the semiconductor layer 220, or by diffusing from agaseous source such as POCl₃. This dopant introduction step may befollowed by a furnace or rapid-thermal drive-in anneal. The second dopedregion 225 may function as, for example, the source region of a DMOS orIGBT device, or the emitter region of a vertical bipolar device, or thecathode of a thyristor.

Other steps may be performed before or after the formation of the seconddoped regions 225. For example, lightly-doped drain regions anddielectric spacers (not shown) may be formed on the gate edges,especially if low-voltage CMOS devices are formed concurrently on thesame wafer. Also, contact regions 250 allowing electrical contact to thefirst doped regions 224 may also be formed. As shown in FIG. 22C, if thefirst doped regions 224 are p-type, the contact regions 250 would beheavily doped P+ regions. For DMOS and IGBT, these contacts wouldfunction as body and collector contacts, respectively.

Turning to FIG. 22D, electrical contacts 228 to the second doped regions225 are formed. These contacts 228 provide external electrical access tothe device. As shown in FIG. 22D, these contacts may simultaneously formelectrical contact to the heavily doped regions 250, thus shorting thesecond doped regions 224 and the first doped regions 224. Thisarrangement may be desired, for example, in IGBT, some DMOS (where thesource and body are permanently shorted), and some (shorted-cathode)thyristor devices. The contact formation process could include, forexample, depositing and planarizing a dielectric film 227, for example,using CVD of silicon dioxide followed by chemical-mechanical polishing.Contact holes could be patterned and etched in this planarizeddielectric layer; these contact holes may be filled with a metal to formthe contacts 228. For this step, it may be preferable to use materialsthat are tolerant to later high temperature processes. For example, itmay be desirable to use a refractory metal, e.g., tungsten, for thecontact fill material. One or more interconnect layers connecting to thecontacts 228 may be formed, by either additive (Damascene) orsubtractive processes. Again, it may be preferable to usehigh-temperature-tolerant materials, for example, refractory metals, forthese interconnect layers. Metal-semiconductor compounds may be formedat the surface 226 of the semiconductor layer 220 prior to formation ofdielectric layer 227 and contacts 228. For example, titanium silicide,cobalt silicide, or nickel silicide may be formed at this surface, inorder to reduce interfacial contact resistances.

In FIG. 22E, a handle layer 229 is bonded to the planarized surface ofthe device structure. The handle layer could be, for example, a siliconwafer, or a quartz, sapphire, or borosilicate glass wafer. The bondingmethod may be, for example, direct silicon oxide or fusion bonding,adhesive, or intermetallic bonding. In FIG. 22F, a portion 230 of thesemiconductor layer 220 is removed from its first surface 223. Aphotoresist layer, or another hard mask may be used to pattern thisremoval. Any common semiconductor etch method may be used; for example,a wet chemical etch, or a dry etch. This removal step exposes a thirdsurface 231 of the semiconductor layer 220. Alternatively, the removalof the portion 230 of the semiconductor layer 220 may be performedwithout a pattern. In this case, the removed portion 230 of thesemiconductor layer 220 extends across the layer's entire area.

In FIG. 22G, the exposed back surface 231 of the device is subjected tosemiconductor processing to form a third doped region 232 in thesemiconductor layer 220. As shown in FIG. 22G, the third doped region232 may be doped to the same conductivity type as the background dopingof the semiconductor layer 220; for example, n-type. This would be thecase, for example, if the third doped region 232 were functioning as thedrain contact region of a DMOS device, or the collector contact regionof a vertical NPN bipolar device. The peak concentration of the thirddoped region 232 may be, for example, at least 10 times higher than thebackground doping of semiconductor layer 220, or, for example, 10¹⁸ to10²¹ cm⁻³, for example, 10¹⁹ to 2×10²⁰ cm⁻³. The doped region 232 may beformed, for example, by implanting phosphorous or arsenic into theexposed back surface 231 of the semiconductor layer 220, or by diffusingfrom a gaseous source such as POCl₃. Alternatively, a dopant such asphosphorus may be implanted through the front surface of the device;thus this dopant introduction step may occur prior to the removal of aportion of the semiconductor layer 220. This dopant introduction stepmay be formed through a patterning layer, for example, a layer ofpatterned photoresist. It may be followed by a furnace or rapid-thermaldrive-in anneal. The dopant introduction or anneal steps may beperformed at temperatures, for example, of 800° C. to 1100° C., forexample, 850° C. to 980° C., and at times from 5 seconds to 1 hour, orfrom 30 seconds to 30 minutes. Such high temperature steps would requirethe use of high-temperature tolerant materials in the formation of, forexample, the front-side contacts 228 and dielectric layer 227.Alternatively, fast anneal methods such as scanning laser anneal orflash anneal could be used to activate the dopant if a material oflesser temperature tolerance is used for the pre-layer-transfermetallization.

In FIG. 22H, a second electrical contact 233 is made to the third dopedregion 232. This electrical contact may be formed, for example, bydepositing a layer of aluminum, or copper, or a refractory metal, andpatterning this layer. Further processing of the device may includedepositing and patterning passivation and pad metal layers, for example.The handle layer 229 may also be removed, if there is enough of theun-etched semiconductor layer 220 to provide support for the structure.

FIG. 23 shows an alternative embodiment of the present invention,wherein the third doped region 234 is doped to the type opposite that ofthe background doping of the semiconductor layer 220. For example, inFIG. 23, the third doped region 234 is heavily doped p-type. This wouldbe the case, for example, if the third doped region 234 were functioningas the collector contact region of an IGBT device, as depicted in FIG.23, or the anode of a thyristor. The doped region 234 may be formed, forexample, by implanting boron into the exposed back surface 231 of thesemiconductor layer 220, or by diffusing from a liquid or gaseous sourcesuch as BBr₃.

FIG. 24 shows another alternative device embodiment. In FIG. 24, thesecond doped regions 224 are contacted from the back surface 231,through diffused regions 252 and electrical contacts 253. Contactregions 253 may be, for example, heavily doped P-type regions. Thestructure shown in FIG. 24 may be useful if, for example, a DMOS devicewith a separate body contact is desired. Alternatively, the structure ofFIG. 24 may be used for an NPN bipolar device, or a thyristor, with thebase contact 253 on the back side of the device.

FIGS. 25A-E show a method of forming an SOI device using back sideprocessing. In FIGS. 25A-E, the semiconductor layer 220 consists of athin SOI layer 240, with a first surface 243 and a second surface(corresponding to the second face of the semiconductor layer 226). Asdescribed in step 1801 for FIG. 18 above, this SOI layer may be, forexample, a layer of silicon 0.2 to 1 to 10 or 20 μm thick. The firstsurface 243 of the SOI layer 240 contacts the first surface 244 of aninsulating layer 241, whose second surface 245 contacts the secondsurface 246 of a substrate 242. The first surface of the substrate 242corresponds to the first surface 223 of the semiconductor layer 220. Theinsulating layer 241 may be, for example, silicon dioxide. The substrate242 may be, for example, a silicon wafer. FIG. 25A shows the SOIvertical device structure just after the front-side contacts 228 havebeen formed.

In FIG. 25B, a handle layer 229 is bonded to the front surface of thestructure, for example, using the same materials and methods describedfor FIG. 22E above. In FIG. 25C, a portion 230 of the substrate layer242 is removed to expose a portion of the insulating layer 241. Any ofthe semiconductor etch methods described above, for example, for FIG.22F, may be used for this etch. Such etch methods may be selective tothe material of the insulator layer 241 (for example, silicon dioxide).In FIG. 25D, the portion of the oxide layer 241 exposed in the previousstep is removed, exposing a portion of the first surface 243 of the SOIlayer 240. This removal may use, for example, HF or an SF₆/O₂ dry etch.

FIG. 25E shows the result of back side semiconductor processing on theSOI structure. The back side processing may be the same processesdescribed above (FIG. 22H; forming, for example, the third doped region232 on the back surface 243 of the SOI layer 240. The electrical contact233 to the third doped region 232 is subsequently formed. Passivationand pad metal deposition and patterning may follow this step. Handlelayer 229 may also be removed, if there is enough mechanical supportprovided by the remaining substrate layer 242.

FIG. 26 shows a variant on the SOI embodiment of FIGS. 25A-E. In FIG.26, the entire substrate 242, as well as the entire insulating layer241, has been removed. In this case, permanent handle layer 229 providesall mechanical support for the device. FIGS. 25E and 26 show a finishedvertical DMOS device with source/body contacts 228, gate layer 222, anddrain contact 233. Other device variations, for example, IGBT, BJT, orthyristors may also be formed using the exemplary SOI method.

The structures of FIG. 22H, 23, 24, 25E, or 26 may also include trenchregions that extend through the semiconductor layer 220. These trenchregions would electrically isolate devices formed in semiconductor layer220 from each other, similar to regions 1616 shown in FIGS. 16A-B. Themetal interconnect forming the electrical contacts 228 could be extendedthrough these trenches, so as to allow electrical contact to all of theterminals of the devices via the back side of the structure, similar tothe scheme shown in FIGS. 16A-B.

Those skilled in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the presentinvention. Nothing in the disclosure should indicate that the presentinvention is limited to systems that are implemented on a single wafer.Nothing in the disclosure should indicate that the present invention islimited to systems that require a particular form of semiconductorprocessing or to integrated circuits. Functions may be performed byhardware or software, as desired. In general, any diagrams presented areonly intended to indicate one possible configuration, and manyvariations are possible. Those skilled in the art will also appreciatethat methods and systems consistent with the present invention aresuitable for use in a wide range of applications encompassing anyrelated to power devices.

While the specification has been described in detail with respect tospecific embodiments of the present invention, it will be appreciatedthat those skilled in the art, upon attaining an understanding of theforegoing, may readily conceive of alterations to, variations of, andequivalents to these embodiments. These and other modifications andvariations to the present invention may be practiced by those skilled inthe art, without departing from the spirit and scope of the presentinvention, which is more particularly set forth in the appended claims.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: providing a semiconductor layer doped to a firstconductivity type, having a first surface and a second surface, forming,in the semiconductor layer and adjacent to the second surface, a firstdoped region of a second conductivity type opposite the first; forminginside the first doped region, a second doped region of the firstconductivity type; forming a first external electrical contact to thesecond doped region at the second surface of the semiconductor layer;removing a portion of the semiconductor layer from the first surface toexpose a new third surface of the semiconductor layer; forming a thirddoped region inside the semiconductor layer adjacent to the new thirdsurface; and forming a second external electrical contact to the thirddoped region at the new third surface of the semiconductor layer.
 2. Themethod of claim 1, wherein the third doped region has the firstconductivity type;
 3. The method of claim 2, wherein the semiconductorlayer is doped uniformly to a first doping concentration, and whereinthe third doped region has a peak doping concentration greater than 10times the first doping concentration.
 4. The method of claim 1, whereinthe third doped region has the second conductivity type.
 5. The methodof claim 1, wherein the third doped region has a doping concentrationgreater than 10¹⁸ cm⁻³.
 6. The method of claim 1, wherein the step offorming a third doped region inside the semiconductor layer comprisesheating the semiconductor layer to a temperature greater than 800° C.for a time longer than 5 seconds.
 7. The method of claim 1, wherein thestep of forming a third doped region inside the semiconductor layeroccurs after the step of removing a portion of the semiconductor layer.8. The method of claim 1, wherein the step of forming a first externalelectrical contact to the second doped region comprises electricallyconnecting the first doped region and the second doped region.
 9. Themethod of claim 1, further comprising, before the step of removing aportion of the semiconductor layer, coupling a handle layer to thesecond surface of the semiconductor layer.
 10. The method of claim 1,wherein the semiconductor layer comprises: an insulator layer having afirst surface and a second surface, a semiconductor-on-insulator layerhaving a first surface and a second surface, the first surfacecontacting the first surface of the insulator layer; and a substratecontacting the second surface of the insulator layer; wherein the stepof removing a portion of the semiconductor layer comprises: removing aportion of the substrate layer to expose a portion of the second surfaceof the insulator layer; and removing a portion of the insulating layerto expose a portion of the first surface of thesemiconductor-on-insulator layer.
 11. The method of claim 10, whereinthe step of removing a portion of the substrate layer comprises removingthe entire substrate layer.
 12. The method of claim 1, wherein the stepof forming an electrical contact to the second doped region at thesecond surface of the semiconductor layer comprises using a refractorymetal to contact the second doped region at the second surface of thesemiconductor layer.
 13. The method of claim 12, wherein the refractorymetal comprises tungsten.
 14. The method of claim 1, further comprising,after removing a portion of the semiconductor layer, forming anelectrical contact to the first doped region at the new third surface ofthe semiconductor.
 15. A semiconductor device comprising: asemiconductor layer having a first surface, a second surface, abackground doped region doped to a first conductivity type, and aremoved portion exposing a third surface; the semiconductor layercomprising: a first doped region of a second conductivity type oppositethe first, adjacent to the second surface, a second doped region of thefirst conductivity type, the second doped region being adjacent to thesecond surface and formed inside the first doped region, a third dopedregion adjacent to the third surface, the third doped region beingseparated from the first surface and separated from the first dopedregion; an electrical contact to the second doped region at the secondsurface; and an electrical contact to the third doped region at thethird surface.
 16. The semiconductor device of claim 15, wherein thethird doped region has the first conductivity type.
 17. Thesemiconductor device of claim 16, wherein the background doped region isdoped uniformly to a first doping concentration, and wherein the thirddoped region has a peak doping concentration greater than 10 times thefirst doping concentration.
 18. The semiconductor device of claim 15,wherein the third doped region has the second conductivity type.
 19. Thesemiconductor device of claim 15, wherein the third doped region has adoping concentration greater than 10¹⁸ cm⁻³.
 20. The semiconductordevice of claim 15, wherein the electrical contact to the second dopedregion electrically connects the first doped region to the second dopedregion.
 21. The semiconductor device of claim 15, further comprising ahandle layer coupled to the second surface of the semiconductor layer.22. The semiconductor device of claim 15, wherein the semiconductorlayer comprises: an insulator layer having a first surface, a secondsurface, and a removed portion; a semiconductor-on-insulator layerhaving a first surface and a second surface, the first surfacecontacting the first surface of the insulator layer, the second surfacecorresponding to the second surface of the semiconductor layer, and theportion of the first surface exposed by the removed portion of theinsulator layer corresponding to the third surface of the semiconductorlayer; and a substrate layer having a first surface, a second surface,and a removed portion, the second surface contacting the second surfaceof the insulator layer, the first surface corresponding to the firstsurface of the semiconductor layer, and the removed portion of thesubstrate layer enclosing or coincident with the removed portion of theinsulator layer.
 23. The semiconductor device of claim 22, wherein theremoved portion of the substrate layer comprises the entire substratelayer.
 24. The semiconductor device of claim 15, wherein the electricalcontact to the second doped region comprises a refractory metal.